(a) Field of the Invention
The present invention relates to tuners, and relates more particularly to a direct broadcasting satellite broad-band tuner with a negative feedback & image compression circuit in which the baseband frequency signal from a demodulator circuit is amplified by a baseband amplifier circuit, then fed back to a variable capacitance diode through a phase correction circuit, which corrects the deviated phase, permitting the DC level of the capacitance diode to be regulated by means of the control of 12 V power supply, then fed back to the voltage control oscillator (VCO) of a microwave monolithic integrated circuit for mixing with the frequency of the radio frequency so that the modulated intermediate frequency signal can be compressed and the compressed image intermediate frequency signal can effectively eliminate noises, increase sound/noise ratio, and improve the threshold when entering said demodulator circuit.
(b) Description of the Prior Art
During the transmission of direct broadcasting satellite, audio and video (baseband frequency) signals are properly arranged at the ground transmitting station through a multiplexer, then modulated to an intermediate frequency signal and then amplified and turned into a radio signal for transmitting through a transmitting antenna. The ground receiving station has a low-noise broad-band radio frequency receiver to receive radio signals. Upon receipt of a radio signal, the signal is amplified and turned into an intermediate frequency signal, and then the intermediate frequency signal is further demodulated by a demodulator circuit to a baseband frequency signal containing a video signal and an audio signal. When the ground receiving station receives a useful electromagnetic wave signal from the atmosphere, interference waves and noises from the atmosphere may interfere with the receiving of the signal or pass into the signal, causing the reliability of the communication affected.
Furthermore, in regular broadband satellite receiving tuners with demodulator means, the intermediate frequency bandwidth is already set, and an IF band pass filter shall be used if to change the bandwidth of the frequency. However, using an IF band pass filter to adjust the bandwidth of the frequency tends to cause a receiving signal loss and image distortion, furthermore it gives little improvement on threshold.